Ophthalmological ultrasonography scanning apparatus

ABSTRACT

An apparatus for ultrasound scanning of the eye is provided comprising a virtual center translocation mechanism that facilitates precise arcuate motion of an ultrasonic transducer to maintain focal distance from the eye and to maintain normality of the ultrasound beam with surfaces of the eye. The invention also provides a radius adjust mechanism for changing the radius of ultrasound scanning to facilitate positioning of the transducer focal point on selected surfaces of the eye. Centration optics are also provided, for aligning the ultrasound transducer with the Purkinje (or other optical or geometric) axis of a patient&#39;s eye.

FIELD OF THE INVENTION

[0001] The invention is in the field of medical ultrasound apparatus,particularly apparatus for use in ultrasonography of the eye.

BACKGROUND OF THE INVENTION

[0002] Ultrasound may be used in a variety of medical applications,including diagnostic ultrasonography of the eye. Diagnostic informationis typically provided by an ultrasound pulse from a piezoelectrictransducer, which is directed into a tissue. Reflected acoustic energyis detected (as ‘echoes’), so that the amplitude of the received energymay be correlated with the time delay in receipt of the echo. Theamplitude of the echo signal is proportional to the scattering strengthof the refractors in the tissue, and the time delay is proportional tothe range of the refractors from the transducer. A variety of hand-heldultrasound instruments for measuring corneal thickness (calledpachymeters) have been developed (for example see U.S. Pat. Nos.4,564,018; 4,817,432; 4,930,512). Many prior art ultrasonic pachymetersprovide A-scan output, in the form of waveforms displayed on a cathoderay tube, representing acoustic reflections in a single dimensional‘column’ of tissue.

[0003] In B-scan ultrasonography, a two-dimensional image is formed, inwhich pixel brightness reflects the amplitude of the reflected acousticsignal. A B-scan image therefore represents a cross-sectional slice ofthe imaged tissue. The cross-sectional information is typically providedby correlating information from a series of adjoining columnar scans(each of which may be used to produce A-scan output). For the purpose ofproducing B-scans, adjoining columnar scans may be produced by a numberof methods: rectilinear translocation of a transducer over the tissue ofinterest; pivoting angular displacement of a single transducer over afan-shaped area; or through the use of a linear array of transducers.

[0004] In some applications, three dimensional images may bereconstructed from a series of B-scans. U.S. Pat. No. 4,932,414 toColeman et al. for example describes a system in which the transducer iselectronically swept or physically rotated to produce a series ofsectored (fan-shaped) scan planes which are separated by a known angulardistance, to produce a 3-dimensional display. In a similar fashion, U.S.Pat. No. 5,487,388 to Rello et al. discloses an ultrasonic scanningsystem in which sequential fan-shaped B-scan image planes are obtainedby movement of the transducer probe in an arc, a movement which allowsthe apex of the scanned 3-dimensional volume to be located below theprobe to facilitate imaging between closely-spaced surface obstructions.

[0005] The structure of the eye, particularly the cornea, presentsspecial problems for optimal ultrasonographic B-scan imaging. The humancornea is an asphere, flattening concentrically, typically approximately11 mm across with an average central radius of curvature of 7.8 mm whichincreases towards the periphery. The high resolution required forultrasonic imaging of some corneal structures is optimally achieved ifultrasound data is collected from the focal point of the transducer, andthe ultrasound beam is normal to the surface of the cornea. As a result,rectilinear scanning of the cornea provides optimal imaging informationonly from relatively small segments of the cornea which are normal tothe transducer beam and in the plane of beam focus. Similarly,volumetric 3-dimensional scanning by reconstruction of a series offin-shaped B-scan planes, as for example described in U.S. Pat. Nos.4,932,414 and 5,487,388, is not a system adapted to provide the degreeof resolution required for biometry of the corneal surface.

[0006] High frequency ultrasound has been used in ophthalmologicalultrasonography to obtain biometric B-scan images of the human cornea,by arcuate translocation of a single element focused transducer.Silverman et al., 1997, J. Ultrasound Med. 16:117-124, describe a systemfor sonographic imaging and biometry of the cornea in which asophisticated programmable motion system permits ultrasonographic arcscanning. In the Silverman et al. system, the ultrasonic transducer istranslated through an arc matched to the approximate radius of curvatureof the cornea using five servo motors and a controller. Similarly, U.S.Pat. No. 5,331,962 discloses an ultrasound system for corneal arcscanning, in which a transducer is translocated along a curved trackthat approximates the surface curvature of the cornea

SUMMARY OF THE INVENTION

[0007] In one aspect of the invention, an apparatus for ultrasoundscanning of the eye is provided comprising a virtual centertranslocation mechanism that facilitates precise arcuate motion of anultrasonic transducer to maintain focal distance from the eye and tomaintain normality of the ultrasound beam with surfaces of the eye. Thearcuate movement of the transducer focal path may closely approximatethe surface of the cornea. Some embodiments of the invention may includea radius adjust mechanism for changing the radius of ultrasoundscanning, to accommodate different eye sizes and to facilitatepositioning of the ultrasound transducer focal point on selectedsurfaces of the eye, such as the cornea Centration optics may also beprovided, for aligning the translocation path of the ultrasoundtransducer with an axis such as, but not limited to, the Purkinje axisof a patient's eye.

[0008] In one embodiment, the invention provides an ultrasoundtransducer support comprising a transducer mount adapted to accommodatean ultrasound transducer having a focal point. The support may beprovided with a virtual centre mechanism attached to the transducermount, for moving the ultrasound transducer along an arcuate translationpath. The arcuate translation path of the transducer may be offset froma virtual centre of translocation by a radius of transducertranslocation, so that the focal point of the ultrasound transducertraverses an arcuate focal path about the virtual centre oftranslocation. A radius adjust mechanism may be provided for adjustingthe position of the transducer mount to change the radius of transducertranslocation.

[0009] In an alternative embodiment, the invention provides a method ofophthamological ultrasonography comprising centring an ultrasoundtransducer having a focal point in alignment with the Purkinje or otheroptical or geometric axis of a patient's eye using centration optics,and moving the ultrasound transducer along an arcuate translation pathintersecting the Purkinje or other optical or geometric axis of thepatients eye. The arcuate translation path of the transducer may beoffset from a virtual centre of translocation by a radius of transducertranslocation, so that the focal point of the ultrasound transducertraverses an arcuate focal path about the virtual centre oftranslocation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1A is a side elevational view of an ultrasound transducersupport of the invention, showing a cam-actuated radius adjustmechanism.

[0011]FIG. 1B is an isometric view of an alternative embodiment of theultrasound transducer support of the invention, showing shaped armlinkages, as are also shown in FIG. 4.

[0012]FIG. 1C is a schematic diagram showing a linking elementconnecting the front and rear swinging linkages which may form part ofthe transducer part of the invention.

[0013]FIG. 2 is a schematic diagram showing the motion of the transducersupport of the invention.

[0014]FIGS. 3A and 3B are elevations views of the embodiment of theinvention shown in FIG. 1, showing the cams that are part of the radiusadjust system in different positions.

[0015]FIG. 4A is a side elevational view showing the ultrasoundtransducer support of the invention with accessory apparatus for sealinga fluid-filled chamber against the patient's eye.

[0016]FIG. 4B is a schematic illustration showing alternative opticswhich may be used in conjunction with methods of centering thetransducer using the apparatus of the invention.

[0017]FIG. 5 is a schematic illustration of a series of meridionalultrasound scanning paths which intersect at a point near the apex ofthe cornea.

[0018]FIG. 6 is an isometric view of a stage for the scanning apparatusof the invention, providing for rotational movement of the scanningapparatus, as well as movement in X, Y and Z axes.

[0019]FIGS. 7 and 7A are cross-sectional side views showing a membranewhich may be used in some embodiments to isolate a volume of fluidaround a patient's eye.

[0020]FIG. 8 is an elevational view showing a mechanical safety stopmechanism.

DETAILED DESCRIPTION OF THE INVENTION

[0021] In one aspect, the invention provides an ultrasound transducersupport comprising a transducer mount adapted to accommodate anultrasound transducer, and a virtual centre mechanism. FIG. 1Aillustrates an embodiment of a virtual center mechanism. First andsecond arm linkages 65A and 65B are each connected via three pivots tomoving parts of the mechanism. Rear swinging pivots 96 connect first andsecond arm linkages 65A and 65B to rear radius adjust slider 71, andrear radius adjust slider 71 is attached to rear swinging linkage 81.Similarly, front swinging pivots 95 connect arm linkages 65A, 65B tofront radius adjust slider 70, and front radius adjust slider 70 isattached to front swinging linkage 80. The front ends of the armlinkages 65A, 65B are connected by transducer pivots 60 to transducermount 55, and transducer mount 55 is adapted to accommodate ultrasonictransducer 50. Front pivot 85 and rear pivot 86 are stationary relativeto the swinging motion of front swinging linkage 80 and rear swinginglinkage 81.

[0022] The virtual centre mechanism is attached to transducer mount 55for moving the ultrasound transducer 50 along an arcuate translationpath 56 offset from a virtual centre of translocation 52 by a radius oftransducer translocation, so that the focal point 51 of the ultrasoundtransducer 50 traverses an arcuate focal path 59 about virtual centre oftranslocation 52. As shown in FIG. 2, when rear swinging linkage 81rotates about rear pivot 86, rear swinging pivots 96 describe arcuatepaths about rear pivot 86. Arm linkages 65A, 65B move front swingingpivots 95, so that front swinging pivots 95 describe identical pathsabout front pivot 85. Similar triangles 53 show that this swingingmotion causes ultrasonic transducer 50 to move in an arc such that itsaxis pivots about virtual center 52. In addition, transducer focus point51 traverses an arc 59 about virtual center 52 at image radius 54. Thepivoting motion of the apparatus may be driven by scanning driver 82,which may for example be a servo motor. It will be seen that focal point51 may also lie behind virtual center 52, for example to scan the backof the eye.

[0023] The mounting of transducer 50 in transducer mount 55 may beadapted so that the position of transducer 50 is adjustable relative totransducer mount 55. Such an adjustment may be difficult to accomplishduring operation, due to the configuration of the assembled apparatus,as shown in FIG. 4. A radius adjust mechanism for adjusting the radiusof transducer translocation may be provided, for example by radiusadjust sliders 70, 71 which are movable relative to the respective pivotpoints 85, 86. In operation, the effect of movement of radius adjustsliders 70, 71 is to elongate similar triangles 53. The elongation oftriangles 53 reflects simultaneous changes to three radii: a ‘first’radius of rotation of front swinging pivots 95; a ‘second’ radius ofrotation of rear swinging pivots 96, and the radius of transducertranslocation circumscribed by transducer pivots 60. In addition, imageradius 54 is changed (the distance between virtual centre 52 and thearcuate focal path 59 traversed by the focal point 51 of transducer 50).The radius adjustment may be driven by rotating radius adjust cams 75,76 relative to swinging linkages 80, 81. Radius adjust cams 75, 76 maybe linked by a rotation linking mechanism, such as anti-backlash belt90, which operates so that adjusting one cam automatically adjusts theother cam by the same amount. Alternatively, a single cam 75 or 76 couldbe used on either slider 70 or 71, in which case the other slider wouldfollow. Mechanisms other than cams, such as motors, gears, or othermechanical linkages may be used to actuate sliding movement of radiusadjust sliders 70, 71.

[0024] To provide extra rigidity to the mechanism supplementary linkingsuch as that shown in FIG. 1C may be used. Linking element 203 may forexample be a steel band or a belt or a chain or a cable and may engagesheaves 201 and 202. Alternatively the linking may be supplied manyother ways including driving both swinging linkages 80 and 81 directlywith wormgears or flexures.

[0025] Ultrasonic transducers for use in accordance with various aspectsof the invention may be high frequency transducers, operating forexample at frequencies between 50 and 100 MHz. A saline bath may be usedto acoustically couple ultrasound transducer 50 to patient's eye 105.FIG. 4A shows the general arrangement of an embodiment of the ultrasoundtransducer support of the invention with accessory apparatus including asaline bath adapted for diagnostic use. In the illustrated embodiment, apatient may be scanned in a seated position by placing the patient'sorbit against eye seal 15. The patient's head may be supported by headsupport 170 which may be adapted to immobilize the patients head duringultrasound scanning. The overall axis of the apparatus, shown as line 25in FIG. 4, may be at an angle of about 45 degrees to horizontal.Alternative angles from horizontal to vertical may also be used. In someembodiments, a patient's mandible may be supported with an upward forcewhich encourages the teeth into mechanical contact to stabilize thepatient's head. Arranging the apparatus at an overall axis of 45 degreesmay help to reduce the accumulation of bubbles in the vacinity of thepatient's orbit, particularly when saline fluid fills reservoir 20 andeye seal 15.

[0026] Coarse alignment of the eye on axis 25 may be done visually, forexample using video camera 140, which preferably has a very highsensitivity. The seal may be tested by slowly filling the saline chamberwith saline and watching for leaks. The position of the patient's headmay be adjusted, or the eye seal changed, in order to achieve a goodseal. Once an acceptable position has been found, the patient's head maybe locked into position by immobilizing the head support. With the headstationary the scanning mechanism 10 can be moved relative to salinechamber 15 to make scan axis 25 coincident with the Purkinjie (or otheroptical or geometric) axis of the patient's eye.

[0027] In accordance with one aspect of the invention, corneal scanningmay be undertaken along a series of meridional paths which intersect ata point near the apex of the cornea, as shown in FIG. 5. In someembodiments, this intersection point may be the Purkinje (or otheroptical or geometric) axis of the eye, which may be used as anapproximation of the optical axis of the eye (defined by the linebetween the object of regard and the fovea of the retina). The Purkinjeaxis may be located by shining a focused beam of light into the patientseye, and examining the Purkinje reflections from four optical surfacesof the eye: the front and rear surfaces of the cornea, and the front andrear surfaces of the lens. The Purkinje reflections are observable alongthe axis of the light beam. The Purkinje axis is located when thereflections from these four surfaces are coincident. A light beam usedto locate the Purkinje axis may also conveniently serve as a view targetfor the patient. Other axes may be used as an intersection point formeridional scanning such as the vertex-fixation axis. When a light isshone axially toward the eye onto the corneal surface, two reflectedimages can be seen—the specular Normal to incident light) reflection andthe diffuse reflection (not necessarily Normal reflection). When theposition of the light source is adjusted such that the specular anddiffuse reflections from the corneal surface are coincident, the lightsource will now be perpendicular to the vertix of the cornea. The vertexfixation axis is obtained when the patient's eye is looking directly ata fixation target, while observing coincidence of the diffuse andspecular corneal surface reflections.

[0028]FIG. 4A shows an embodiment that includes accessory centrationoptics for centering the transducer in alignment with the Purkinje axisof the patients eye. Centration light source 120 may be refined usingaperture 126 and focused using centration optics 125. Centration lightsource 120 may for example be a laser, laser diode, light emitting diodeor incandescent source. The centration light beam may be aligned withmachine axis 25 using reflector 130, such as a prism or mirror, and beamsplitter 135. The centration light beam then passes through fluid-sealedcamera window 136 and through the fluid (saline) in cavity 175 beforereaching the patient's eye 105. As shown in FIG. 4B in order to addresspotential back reflection problems from window 136, both camera 140 andwindow 136 may be tipped relative to machine axis 25 in such a way thatthe centration beam still travels along the machine axis 25 within thesaline chamber 175. The centration light beam thereby intersects thearcuate translation path of transducer 50. The Purkinje reflections thenreturn back through beam splitter 135 and may be recorded by camera 140through lens 145. As shown in FIG. 4, in order for the light to reachthe patient's eye 105, transducer 50 must be swung over to the side asshown in FIG. 2. During an ultrasound scan, because the centration lightbeam intersects the arcuate translation path of transducer 50, thepatient using the centration light as a view target will see the lightdisappear momentarily as the light is blocked by the passing transducer.This flashing behavior may be helpful in facilitating alignment of theeye, since the photoreceptors in the retina would otherwise saturateafter a few seconds of staring at a fixed target light which may causethe eye to shift slightly to compensate.

[0029]FIG. 4A also illustrates focus point illuminator 155, which shinesthrough focus point optics 160 and aperture 161 to produce a focus pointspot on eye 105. The angle of focus point illuminator 155 is set so thatwhen the focus point spot is appropriately positioned on the eye, thetransducer apparatus is in a selected vertical position at a knowndistance from eye 105. The centration optics may for example be used todetermined when the focus point spot joins the Purkinje (or other axis)reflections from the centration light 120. In some embodiments, thispositioning of the focus point spot may be used to identify the point atwhich the apparatus of the invention is positioned at the correctdistance from the eye to have the cornea within the focal point oftransducer 50.

[0030] For extra illumination to improve the eye image on camera 140, aninfra-red light may be shone through either of windows 136, 150, inwhich case the camera will be adapted to be sensitive to the wavelengthselected.

[0031] In addition to the scanning motion shown in FIG. 5, several othermotions may be produced by the mechanism of the invention to scan aneye. In order to produce various meridian angles theta as shown on FIG.5, the scan mechanism 10 may rotate about the machine axis 25 (shown inFIG. 4). Rotational motion of the scanning apparatus may be accomplishedusing rotary table 210. Motion in the Z axis, which shifts the mechanismtoward or away from the eye, may be used to compensate for the degree ofinsetting of a patients eye. Motion in the Z axis may be accomplishedusing a Z-slide 215, which may be motorized or manually controllable.Motion along the X and Y axes, perpendicular to the machine axis 25, maybe used to adjust the position of the ultrasound scanning apparatus oncea patient has been positioned in front of the machine. These motions maybe produced by X slide 220 and Y slide 225. In some embodiments, the Xand Y slides may be motorized to facilitate X and Y motion of thescanning apparatus in planar scans of eye structures, such as the irisplane. These axes may of course be arranged differently than shown inFIG. 6 while retaining the same essential operation.

[0032] In order to provide a mechanical means of preventing thetransducer from approaching an eye too closely, a safety stop as shownin FIG. 8 may be used. The transducer may be shifted closer to the eyeby either a radius adjustment or Z axis adjustment. A curved stop bar212 may be fixed to the body of the Z axis stage 215. Stop pads 210 and211 are fixed to radius adjust slider 205 so that an excess motion ofeither the radius or Z axes causes one of the pads to touch the stopbar. These stop pads 210, 211 may be supplemented with sensors foroperator feedback.

[0033] In some embodiments, it may be desirable to provide a barrier toinhibit the passage of an infection from one patient to another. In someembodiments, it will be necessary for the centration light beam and thePurkinje (or other axis) reflections to pass through such a barrierwithout significant shifting or distortion. In one embodiment, membrane180 as shown in FIG. 7 may be used, which has saline fluid on both sidesof it and is selected to have a similar index of refraction to saline sothat light rays passing through membrane 180 will be affected verylittle by its presence. A filling and draining system may be provided,as shown by tube 181 in FIG. 7. The outer edges of the membrane 10 maybe draped over the eye seal and provide the sealing surface for theface. Near its center membrane 180 may be attached by clamp 190 totransducer 50. Clamp 190 may be adapted to accommodate rotation oftransducer 50 relative to the eye seal 15 during a scan, for example bypermitting rotational movement of transducer 50 within clamp 190.Alternatively, membrane 180 may be continuous, and adapted to permittransmission of ultrasonic vibrations through the membrane itself asshown in FIG. 7A. In some embodiments, bellows seal 173 may be providedover ultrasound transducer 50 and linkage arms 65A, 65B.

[0034] Although various embodiments of the invention are disclosedherein, many adaptations and modifications may be made within the scopeof the invention in accordance with the common general knowledge ofthose skilled in this art. Such modifications include the substitutionof known equivalents for any aspect of the invention in order to achievethe same result in substantially the same way. Numeric ranges areinclusive of the numbers defining the range. In the claims, the word“comprising” is used as an open-ended term, substantially equivalent tothe phrase “including, but not limited to”.

What is claimed is:
 1. An ultrasound transducer support comprising: a) atransducer mount adapted to accommodate an ultrasound transducer havinga focal point; b) a virtual centre mechanism attached to the transducermount for moving the ultrasound transducer along an arcuate translationpath offset from a virtual centre of translocation by a radius oftransducer translocation, so that the focal point of the ultrasoundtransducer traverses an arcuate focal path about the virtual centre oftranslocation; and, c) a radius adjust mechanism for adjusting theposition of the transducer mount to change the radius of transducertranslocation.
 2. The ultrasound transducer support of claim 1, whereinthe virtual centre mechanism comprises: a) first and second arm linkagesconnecting the transducer mount to front and rear swinging linkages, thefront swinging linkage being mounted for rotational movement about afront pivot, the rear swinging linkage being mounted for rotationalmovement about a rear pivot, wherein: i) the first and second armlinkages are connected to the transducer mount by transducer pivots; ii)the first and second arm linkages are connected to the front swinginglinkage by front swinging pivots; iii) the first and second arm linkagesare connected to the rear swinging linkage by rear swinging pivots. 3.The ultrasound transducer support of claim 2, wherein the front swingingpivots are radially spaced apart equidistant from the front pivot on thefront swinging linkage, and the rear swinging pivots are radially spacedapart equidistant from the rear pivot on the rear swinging linkage, sothat when the front swinging linkage rotates about the front pivot: a)the front swinging pivots traverse a first circular arc which is a firstradius from the front pivot; b) the rear swinging linkage rotates aboutthe rear pivot so that the rear swinging pivots traverse a secondcircular arc which is a second radius from the rear pivot; and, c) thetransducer pivots traverse the arcuate translation path about thevirtual centre of translocation, the arcuate translation path beingoffset from the virtual centre of translocation by the radius oftransducer translocation, wherein the first radius, the second radiusand the radius of transducer translocation are the same magnitude. 4.The ultrasound transducer support of claim 3, wherein the radius adjustmechanism is adapted to simultaneously vary the first radius, the secondradius and the radius of transducer translocation, the radius adjustmechanism comprising: a) a front radius adjust slider slidably mountedon the front swinging linkage, with the front swinging pivots mounted onthe front radius adjust slider; b) a rear radius adjust slider slidablymounted on the rear swinging linkage, with the rear swinging pivotsmounted on the rear radius adjust slider; wherein the front and rearradius adjust sliders are operably linked so that sliding movement ofthe front and rear radius adjust sliders with respect to the front andrear swinging linkages simultaneously changes the first radius, thesecond radius and the radius of transducer translocation.
 5. Theultrasound transducer support of claim 4, wherein the radius adjustmechanism further comprises a cam for actuating sliding movement of thefront and rear radius adjust sliders with respect to the front and rearswinging linkages.
 6. The ultrasound transducer support of claim 1,wherein the ultrasound transducer is a single element focusedtransducer.
 7. The ultrasound transducer support of claim 1, wherein thearcuate focal path is between the virtual centre of translocation andthe ultrasound transducer.
 8. The ultrasound transducer support of claim1, wherein the virtual centre of translocation is between the arcuatefocal path and the ultrasound transducer.
 9. The ultrasound transducersupport of claim 1, wherein the ultrasound transducer is adjustablymounted in the transducer mount, so that adjustment of the position ofthe transducer in the transducer mount changes the arcuate focal path.10. The ultrasound support of claim 1 further comprising centrationoptics for centring the ultrasound transducer in alignment with anoptical or geometric axis of a patient's eye.
 11. The ultrasound supportof claim 10, wherein the axis of the patient's eye is the Purkinje axis.12. The ultrasound support of claim 10, wherein the centration opticscomprises a centration light source having a centration light beamalignable to intersect the arcuate translation path of the transducer.13. An ultrasound transducer support comprising: a) a transducer mountadapted to accommodate an ultrasound transducer having a focal point; b)a virtual centre mechanism attached to the transducer mount for movingthe ultrasound transducer along an arcuate translation path offset froma virtual centre of translocation by a radius of transducertranslocation, so that the focal point of the ultrasound transducertraverses an arcuate focal path about the virtual centre oftranslocation c) centration optics for centring the ultrasoundtransducer in alignment with the an optical or geometric axis of apatient's eye wherein the centration optics comprise a centration lightsource having a centration light beam alignable to intersect the arcuatetranslation path of the transducer.
 14. The ultrasound transducersupport of claim 13, wherein the optical or geometric axis of thepatient's eye is the Purkinje axis.
 15. The ultrasound support of claim1, further comprising a focus point illuminator adapted to produce afocus spot appropriately positioned on a patient's eye when theultrasound transducer is a known distance from a patient's eye.
 16. Amethod of ophthamological ultrasonography comprising: a) centering anultrasound transducer having a focal point in alignment with an opticalor geometric axis of a patient's eye using centration optics; b) movingthe ultrasound transducer along an arcuate translation path intersectingthe optical or geometric axis of the patient's eye, wherein the arcuatetranslation path is offset from a virtual centre of translocation by aradius of transducer translocation, so that the focal point of theultrasound transducer traverses an arcuate focal path about the virtualcentre of translocation.
 17. The method of ophthamologicalultrasonography of claim 16, wherein the optical or geometric axis ofthe patient's eye is the Purkinje axis.
 18. The method ofophthamological ultrasonography of claim 16 further comprising the stepof adjusting the radius of transducer translocation.